Method and apparatus for displaying coherent light images

ABSTRACT

A method and apparatus for destroying the scintillation or speckling effect observed when a bema of coherent light is impinged or directed upon a stationary screen includes a relatively thin layer of an organic nematic mesomorphic compound which is impressed with a voltage above the particular threshold electric field which causes the compound to scatter light.

Unite States Baker 1 51 Mar. 21,1972

[54] METHOD AND APPARATUS FOR DISPLAYING COHERENT LIGHT IMAGES [72}Inventor:

[73] Assignee:

Charles E. Baker, Dallas, Tex.

Texas Instruments Incorporated, Dallas, Tex.

[22] Filed: Dec. 23, 1969 [21] Appl.N0.: 887,511

52 11.5. c1 ..350/320, 40/52, 352/40, 353 69, 353/121, 350/117, 350/16051 1111.01. ..G03b 21/60 58 Field ofSearch ..350/160, 117, 123, 320;40/52, 40/l06.52; 352/40, 43, 201, 244; 353/28, 46, 69, 121

[56] References Cited UNITED STATES PATENTS 3,322,485 5/1967 Williams..350/160 3,425,771 2 1969 McCown 350/160 3,499,702 3 1960 Goldmacher eta1. .350/160 3,524,145 8/1970 Fowler 330/94 5 3.551,026 12/1970Heilmeier. ...350/160 3,576,364 4/1971 Zanoni ..350 160 MODULATOR OTHERPUBLICATIONS Liquid Crystals...," The Glass lndustry 8/68 pp. 423- 425.Dakss et a1. Acousto-Electro-Optical Scanlaser, l.B.M. Tech. Discl.Bul., V01. 11, No.5 10/68 pp. 532- 534.

Fowler et al., A Survey of Laser Beam Deflection Techniques," AppliedOptics, Vol. 5, No. 10 pp. 1675- 1681, 10/66 B. M. Elson, ExperimentalDisplay Devices Use Films of 7, Liquid Crystals," Aviation Weelg & SpaceTech. July 8, 1968,

Primary Examiner-Ronald L. Wibert Assistant Examiner-Jeff RothenbergAtt0rney-Samue1 M. Mims, Jr1, James 0. Dixon, Andrew M. Hassell, HaroldLevine, Rene E. Grossman, Mel Sharp and John Vandigriff [57] ABSTRACT Amethod and apparatus for destroying the scintillation or specklingeffectobserved when a bema of coherent light is impinged or directedupon a stationary screen includes a rela' tively thin layer of anorganic nematic mesomorphic compound which is impressed with a voltageabove the particular threshold electric field which causes the compoundto scatter light.

8 Claims, 2 Drawing Figures SCREEN METHOD AND APPARATUS FOR DISPLAYINGCOHERENT LIGHT IMAGES This invention relates to optical displays and,more particularly, to an electro-optical display apparatus and methodfor use with a coherent light beam.

When a visible coherent light beam, for example, from a laser,illuminates a fixed diffuse reflecting surface such as a matte whitescreen, the illuminated area has a sparkling appearance. The sameobservation can be made when such a coherent light beam is directed ontoa stationary diffuse optical transmission surface such as a rearprojection display screen. Near the display surface, when the eye isfixed, the illuminated area appears to be peppered with bright and darkdots'lik'e a beaded screen'or a very grainy photograph. As the viewingdistance increases, the dots grow in size and the surface takes on amottled or speckled appearance. At still greater distances, singlebright or dark patches cover the entire illuminated area causing it toflicker or twinkle as the head is moved from side to side. Stopping downthe pupil of the eye causes the spots to grow and the twinkling to occurat a closer range. Lateral motion of the head causes the spots to movewith respect to the screen. Near the illuminated area, the spots movewith the head as if they were behind the screen. At greater distances,some observers see them move with the head while others see them move inthe opposite direction as if they were in front of the screen. Thesephenomena are commonly known as the scintillation effect or specklingeffect caused by the incidence of coherent light on a diffuse surface.This phenomenon has been attributed to the fact that coherent lightreflected by or through a diffusing surface produces a complex, random,but stationary diffraction pattern.

Several methods for destroying this scintillation or speckling effecthave been disclosed in the art. A coherent light source has been rotatedin a continuous manner about the optical axis of the system to partiallydestroy the coherence between the source and the object. Diffusescreens, as the ones used in relation to the explanation above, havebeen utilized to destroy the scintillation effect by setting them in aconstant vibratory motion, thus, breaking up the stationary diffractionpattern caused by the coherent light. Another method used foreliminating the speckling has been to pass the laser beam through a weakmilk-water solution. In the latter method, the suspended milk particlesundergoing Brownian motion provide a time varying diffusing screen whichyields results comparable to those obtained with incoherentillumination. A time averaging technique on a spherical wave front hasbeen accomplished by arranging for the incident wave of coherent lightto be passed through a rotating flat glass plate, tilted slightly out ofthe plane perpendicular to the optical axis, prior to striking aprojection surface. All of the foregoing techniques for destroying thescintillation effect have drawbacks either in the fact that they areonly experimentally practical or that they add greatly to the cost andcomplexity ofthe optical display equipment utilizing coherent lightsources.

It is therefore desirable to eliminate this stationary, random, complexdiffraction pattern which results when a coherent light source isdirected toward a diffuse display screen. It is desirable to possess adisplay screen or apparatus which will in itself destroy or eliminatethe speckling observed when coherent light from a source such as, forexample, a laser is directed upon a screen to display information.Development of a mechanically simple and relatively inexpensive screenfor a television type, rear projection laser display is of increasingimportance since such types of displays are becoming of increasingimportance, especially for use as a large screen dynamic display.

This invention therefore provides a novel apparatus comprising a sourceof a coherent light beam, a screen means for scattering a coherent lightbeam, means for directing the light beam toward the screen means, thescreen means including an optically transmissive layer of an organicnematic mesomorphic compound less than mils thick and means forcontinuously impressing a voltage gradient across a functional portionof the layer having a value greater than the threshold value whichcauses the compound to scatter light. The procedure for carrying out thepresent invention includes displaying a coherent light beam on a screencomprising directing a coherent light beam toward an opticallytransmissive, relatively thin layer of an organic nematic mesomorphiccompound having a threshold electrical field which when exceeded causesthe compound to scatter light, and impressing a voltage gradient acrossthe layer of the organic nematic mesomorphic compound.

For a better understanding of preferred embodiments of the presentinvention, refer to the attached drawings wherein:

FIG. 1 is a schematic illustration'of a coherent light beam directedtoward a display apparatus of the present invention;

FIG. 2 is a schematic illustration of another embodiment of the presentinvention utilizing a display apparatus of the present invention.

Referring now to FIG. 1, a coherent light source 10, illustratedschematically as a laser, directs a coherent light beam 12 through lightmodulator 14 in which the intensity or brightness of the light beam isvaried in accordance with a predetermined input signal. From the lightmodulator 14 the intensity modulated laser beam 16 is directed into ascanning mechanism or light beam deflector which causes the light beamto be deflected or scanned in a predetermined pattern. This pattern isprojected or displayed upon a screen, generally designated 18. Suitablelaser light modulating mechanisms and light beam deflector apparatus aredisclosed in -Laser Display Technology" by C. E. Baker, IEEE Spectrum,Vol. 5, No. 12, Dec., 1968, pages 3950, incorporated herein byreference. The rear projection display screen 18 includes a firsttransparent substrate 20, for example a planar sheet of glass. Thesubstrate is coated on one side with a layer or coating 22 of materialwhich is optically transmissive and electri cally conductive. Thedisplay screen also includes a second substrate 24 similar to substrate20 which is also coated with a layer or coating 26 of material which iselectrically conductive, and in this embodiment also opticallytransmissive. Sand wiched between the two layers 22 and 26 and inelectrical contact therewith, is a relatively thin layer 30 of anorganic nematic mesomorphic compound. The layer has an optimum thicknessofless than 20 mils, preferably less than 10 mils, and most preferablyhas a thickness in the range of l to 4 mils. Leads 32 and 34 areconnected respectively to the electrically conductive layers 22 and 26.The leads are also connected to a power source 36 which impresses asufficient voltage when energized to cause the organic nematicmesomorphic compound to diffuse light which strikes it, as will beexplained hereinafter. The power source is preferably a direct currentsource, although alternating current is usable.

To the eye of the observer then, the beam of coherent light 38 strikingthe rear side of the projection screen 18 appears to be a brightstationary display point, not exhibiting the heretofore troublesomephenomenon of speckling or scintillation. The image created on thescreen, of course, can be of a variety of configurations, depending uponthe particular manner in which the coherent light beam is modulatedand/or deflected. An exemplary image can be produced by a raster scan ofthe screen, displaying, for example, a dynamic image such as aconventional line television image. As previously mentioned, this typeof display would be of great importance in a large screen dynamicdisplay such as that utilized in closed circuit television, conferences,and public information displays. Heretofore, this type of display hasbeen subject to the scattering or scintillation effect which increaseswith the distance of the observer from the display screen. In additionthe screen of the present invention can be utilized to improve othercoherent and light image applications, such as transparency projectionand holography.

In FIG. 2, a second embodiment of the present invention is schematicallyillustrated wherein a front projection screen, generally designated 40,is shown rather than a rear projection screen as is illustrated in theprevious figure. Similar reference numerals are utilized whereapplicable. Again, a coherent light beam 12 from, for example, a laseris modulated and deflected according to a predetermined pattern. In thisembodiment, the display screen 40 is similar to that previouslydescribed except that the layer or coating 42 on substrate 44 isoptically reflective rather than optically transmissive. In thisembodiment, the modulated and deflected coherent light beam 46 isdirected towards the display screen 40, is transmitted through substrate20, layer 22 and the layer 30 of the organic nematic mesomorphiccompound where it strikes the reflective coating 42 and is reflectedback toward the eye of the observer. In the same manner as in theprevious embodiment, the coherent light beam is dynamically diffused orscattered resulting in a stationary, nonspeckled or nonscintillatingimage.

The display screen, as disclosed herein, generally must include twosubstrates with coatings of uniform thickness which are spacedsubstantially equidistant along the entire surface which will be scannedor used for display. Among other things, this aids in control of thethickness of the organic nematic mesomorphic compound layer and alsoassists in maintaining substantially an equal voltage drop across theentire layer. The voltage preferably is continuously impressed acrossthe organic nematic mesomorphic layer to effect a continuous stationaryimage as seen by an observer. The substrates comprising the displayscreens 18 or 40, as the case may be, are positioned less than 20 mils,preferably less than 10 mils, and most preferably less than 4 mils apartby means of spacers (not shown) or other suitable spacing means, such asa frame.

The substrates 20 and 24 can be composed of any of a variety ofoptically transmissive solids, for example, the various types of glass,fused quartz, transparent varieties of corundum and transparent plasticsor resins. The term optically transmissive as used herein includes bothtransparent and translucent materials. The transmissive coating on thesubstrates is also transparent in the case of coatings 22 and 26. Such acoating can be composed of layers of indium oxide or tin oxide acrossthe entire surface of the substrate 20 which is contacting the organicnematic mesomorphic compound. If a reflective surface is desired, suchas coating 42, a thin layer of a metallic coating such as silver oraluminum, can be deposited on the substrate 44. When a reflectivecoating is used, it is obviously unnecessary to employ a transmissivesub strate.

As for the organic nematic mesomorphic compound usable within the scopeof the present invention, the three common states of matter or states ofaggregation are the solid, liquid, and the gas, in which the randomnessof geometrical arrangement of the molecule increases from the solid tothe liquid to the gas. The gas and the ordinary liquid are bothisotropic, having the same physical properties in all directions. Thesolid may be amorphous, in which case the geometrical arrangement of themolecules is random, but the molecules are sufficiently bound so theyare unable to move with respect to each other. Most inorganic andorganic solids are found to be crystalline, that is, their molecularunits are arranged in a regular repeating geometric pattern known as alattice unit. Many crystalline solids are anisotropic, since theirphysical properties vary depending on the direction of measurement withrespect to the different crystal axes. For most pure substances, thetransition temperature between the solid state and the liquid state isquite sharp, so that it has become customary to regard the sharpness ofthe melting point ofa solid material as an indication of the purity ofthe material.

Certain organic solid compounds such as esters of cholesterols exhibit apeculiar behavior when heated. These substances melt sharply at onetemperature, but produce a turbid melt. On further heating to a highertemperature, the turbid melt abruptly changes to a clear isotropicliquid. In the temperature range between the solid and the isotropicliquid, these materials are anisotropic with respect to light whenviewed between crossed polaroids. The characteristics of these materialsare thus partly those of the crystalline solid, since they areanisotropic, and partly those of the isotropic liquid, since theyexhibit liquid flow. The materials are therefore often called liquidcrystals," or, more accurately, crystalline liquids. At present, it iscustomary to regard these materials as forming a fourth state of matterknown as the mesomorphic state or mesophase, since it is a state orphase intermediate between that of the anisotropic crystal and that ofthe isotropic liquid.

There are several mesomorphic states or forms, such as the semecticmesophase and the cholesterolic mesophase. One of these is the nematicmesophase. The term nematic means thread-like," since materials in thisstate frequently assume a characteristic thread-like texture when a thinsection of the material is viewed between crossed polaroids. When anematogenic crystalline solid is heated in a capillary tube, thecrystals collapse sharply at the melting point, but in this case form aflowing turbid liquid which uniformly fills the capillary to aparticular height, and exhibits a definite meniscus. At a highertemperature, the turbidity of this nematic mesophase suddenlydisappears, and the liquid becomes an ordinary isotropic liquid. Anexample of a compound which exhibits a nematic mesophase is4'-methoxybiphenyl-4 carboxylic acid. Some organic compounds, forexample, 5 chloro-6-n-heptyloxy-2 naphthoic acid, may exist in eitherthe anisotropic crystalline solid phase, or the anisotropic semectic (adifferent) mesophase, or the anisotropic nematic mesophase, or theisotropic ordinary liquid phase, depending on the temperature. It isbelieved that in the nematic mesophase the molecules of the melt areordered so that the molecules are parallel, but the ends of themolecules are not in any definite or regular arrangement.

Mesomorphic states are generally thermotropic, that is, they areexhibited only within a certain temperature range above the meltingpoint of the solid, and hence are restricted to melts. It has been foundthat when certain solid compounds are dissolved in a suitable solvent,the resulting solution is not a true solution, since it is not isotropicwith respect to the transmission of light. The alkali metal salts oflong chain aliphatic acids tend to exhibit such behavior. When an excessof the solvent is added, these anisotropic solutions become trueisotropic solutions. Certain inorganic materials such as vanadiumpentoxide also exhibit this type of behavior when dissolved in asuitable solvent. These characteristics of inorganic materials arethought to be connected with the colloidal state. Since the anisotropyof these materials depends on their being suitably dissolved, and existsat temperatures below the melting point of the solid, this type ofmesomorphism is known as lyotropic (as opposed to thermotropic)mesomorphism.

For a more complete discussion of the mesomorphic states or mesophases,see, for example, G. W. Gray, Molecular Structure and the Properties ofLiquid Crystals, Academic Press, New York, 1962.

Mesomorphic materials suitable for the practice of this invention areorganic thermotropic nematic compounds. Examples of such compounds are:

deca-2,4-dienoic acid 4,4 di-n-heptoxyazoxybenzene 4,4di-n-hexoxyazoxybenzene 4,4 di-n-pentoxyazoxybenzene 4,4di-n-butoxyazoxybenzene 4,4-diethoxyazoxybenzene undeca-2,4-dienoic acidnona-2,4-dienoic acid 4,4-dimethoxystilbene2,5-di(p-exthoxybenzylidene)cyclopenthanone2,7-di-(benzylideneamino)fluorene 2,p-methoxybenzylideneaminophenanthrene 4-methoxy-4"-nitro-p-terphenyl p-Azoxyanisole4-p-methoxybenzylideneaminobiphenyl 4,4-di-(benzylideneamino)biphenylp-n-Hexylbenzoic acid p-n-Propoxybenzoic acid trans-p-methoxycinnamicacid 6-methoxy-2-naphthoic acid it will be understood that the screens18 or 40 are maintained in the temperature range in which the compoundex hibits the nematic mesophase, i.e., the temperature range above themelting point of the solid compound but below the temperature at whichthe molten compound becomes isotropic. It will also be understood thatthroughout this specification and in the claims, when reference is madeto an organic nematic thermotropic compound, the compound is in thatparticular temperature range in which the nematic mesophase isexhibited. The threshold value for the voltage gradient impressed acrossor electrical field applied to the particular organic nematicthermotropic compound varies with the compound itself and with theparticular distance between the two electrodes. However, for most of theorganic nematic ther motropic compounds, the applied voltage is of theorder of magnitude of about 1,000 volts per centimeter.

It will be apparent that various modifications and alterations can bemade to the foregoing invention without departing from the conceptthereof. The invention is to be limited only by the definition containedin the appended claims wherein:

What is claimed is:

1. A method for producing a specklefree image from a coherent source ona screen, said screen comprising first and second electrodes and aliquid crystal having dynamic light scattering properties when subjectedto a voltage gradient disposed therebetween, said first electrode beingoptically transmissive and said second electrode being opticallyreflecting, the method comprising the steps of:

a. directing a coherent modulated and deflected light beam onto saidscreen, said screen being positioned such that said light beam firstpasses through said transparent electrode, passes through said liquidcrystal and impinges on said second electrode;

b. impressing a voltage on said electrodes to produce a voltage gradientacross said liquid crystal, said voltage gradient being of sufficientmagnitude to cause said liquid crystal to dynamically scatter saidcoherent modulated and deflected light beam thereby producing aspecklefree image on said screen.

2. A method for producing a speckle-free image on a screen in accordancewith claim I, wherein said liquid crystal is an organic nematicmesomorphic compound.

3. A method for producing a speckle-free display on a screen inaccordance with claim 1 wherein said voltage is a DC voltage.

4. A method for producing a speckle-free display on a screen inaccordance with claim 1 wherein said voltage is an alternating voltage.

5. A method for producing a speckle-free image from a coherent source ona screen, said screen comprising first and second optically transmissiveelectrodes and a liquid crystal having dynamic light scatteringproperties when subjected to a voltage gradient disposed therebetween,said method comprising the steps of:

a. directing a coherent modulated and deflected light beam onto saidscreen, said screen being positioned such that said light beam passesthrough said first electrode. passes through said liquid crystal andthrough said second elec trode;

b. impressing a voltage on said electrodes to produce 2 voltage gradientacross said liquid crystal, said voltage gradient being of sufficientmagnitude to cause said liquid crystal to dynamically scatter saidcoherent modulated and deflected light beam thereby producing a specklefree image on said screen.

6. A method for producing a speckle-free image on a screen in accordancewith claim 5, wherein said liquid crystal is an organic nematicmesomorphic compound.

7. A method for producing a speckle-free display on a screen inaccordance with claim 5 wherein said voltage is a DC voltage.

8. A method for producing a speckle-free display on a screen inaccordance with claim 5 wherein said voltage is an alternating voltage.

2. A method for producing a speckle-free image on a screen in accordance with claim 1, wherein said liquid crystal is an organic nematic mesomorphic compound.
 3. A method for producing a speckle-free display on a screen in accordance with claim 1 wherein said voltage is a DC voltage.
 4. A method for producing a speckle-free display on a screen in accordance with claim 1 wherein said voltage is an alternating voltage.
 5. A method for producing a speckle-free image from a coherent source on a screen, said screen comprising first and second optically transmissive electrodes and a liquid crystal having dynamic light scattering properties when subjected to a voltage gradient disposed therebetween, said method comprising the steps of: a. directing a coherent modulated and deflected light beam onto said screen, said screen being positioned such that said light beam passes through said first electrode, passes through said liquid crystal and through said second electrode; b. impressing a voltage on said electrodes to produce a voltage gradient across said liquid crystal, said voltage gradient being of sufficient magnitude to cause said liquid crystal to dynamically scatter said coherent modulated and deflected light beam thereby producing a speckle-free image on said screen.
 6. A method for producing a speckle-free image on a screen in accordance with claim 5, wherein said liquid crystal is an organic nematic mesomorphic compound.
 7. A method for producing a speckle-free display on a screen in accordance with claim 5 wherein said voltage is a DC voltage.
 8. A method for producing a speckle-free display on a screen in accordance with claim 5 wherein said voltage is an alternating voltage. 